users can provide optional expression cache

Gemfile

@@ -8,6 +8,7 @@ end
 gemspec
 
 gem "base64"
+gem "lru_redux"
 
 group :benchmark, :test do
   gem 'benchmark-ips'
@@ -28,5 +29,3 @@ group :test do
     gem 'liquid-c', github: 'Shopify/liquid-c', ref: 'main'
   end
 end
-
-gem "lru_redux"

lib/liquid/expression.rb

@@ -89,7 +89,9 @@ def parse(markup, ss = StringScanner.new(""), cache = nil)
 
         # Cache only exists during parsing
         if cache
-          cache.fetch(markup) { inner_parse(markup, ss, cache).freeze }
+          return cache[markup] if cache.key?(markup)
+
+          cache[markup] = inner_parse(markup, ss, cache).freeze
         else
           inner_parse(markup, ss, nil).freeze
         end

lib/liquid/parse_context.rb

@@ -15,7 +15,7 @@ def initialize(options = Const::EMPTY_HASH)
       # constructing new StringScanner in Lexer, Tokenizer, etc is expensive
       # This StringScanner will be shared by all of them
       @string_scanner = StringScanner.new("")
-      @expression_cache = LruRedux::Cache.new(10_000)
+      @expression_cache = options[:expression_cache] || LruRedux::Cache.new(1000)
 
       self.depth   = 0
       self.partial = false

test/integration/expression_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'test_helper'
+require 'lru_redux'
 
 class ExpressionTest < Minitest::Test
   def test_keyword_literals
@@ -54,6 +55,27 @@ def test_quirky_negative_sign_expression_markup
     )
   end
 
+  def test_expression_cache
+    skip("Liquid-C does not support Expression caching") if defined?(Liquid::C) && Liquid::C.enabled
+
+    cache = LruRedux::Cache.new(10)
+    template = <<~LIQUID
+      {% assign x = 1 %}
+      {{ x }}
+      {% assign x = 2 %}
+      {{ x }}
+      {% assign y = 1 %}
+      {{ y }}
+    LIQUID
+
+    Liquid::Template.parse(template, expression_cache: cache).render
+
+    assert_equal(
+      ["1", "2", "x", "y"],
+      cache.to_a.map { _1[0] }.sort,
+    )
+  end
+
   private
 
   def assert_expression_result(expect, markup, **assigns)